I've been trying to make a new theme in the last days and managed to change the sliders the way I want them to. The only problem is that the handle sizes are the same for the volume slider and the much wider time slider.Now, you can define a style for an object name, e.g.:

which works for the controls defined in the .ui files but I had no luck finding the object names for the two sliders in the source code (note: I'm a very much a newbie in C++ and QT). "MySlider::" works, but naturally for both sliders together.

OK, so apparently no object name is fixed in the code (???, I actually don't have a clue how these things work in C++). So I've tried adding one under VolumeSliderAction::createWidget in the widgetactions.cpp and it works, apparently even if you add two of the same sliders (once again, I'm surprised).

However, I've also found a workaround without compiling by specifying the toolbar in the stylesheet:For sliders in the main toolbar: